This report is a revised version of a paper developed originally for the Conventional Defense Study Group (CDSG) created by the Congress under the 1988-198 National Defense Authorization Act. The CDSG is chaired by the Comptroller General and has representatives from the Congressional Budget Office, Congressional Research Service, Office of Technology Assessment, and General Accounting Office. The author was asked to develop, and present at a workshop, a paper on a net assessment of the NATO/Pact Central Region balance that would include issues of quality, combat readiness, mobilization, and sustainability. The original paper was published in April 1988 as RAND P-7427. The changes incorporated here include editorial improvements, clarification of NATO's sustainability problems, and a better description of the analytic differences among competing approaches to balance assessment. A basic question in any discussion of the military balance is which balance one is addressing: the balance of inputs such as defense expenditures and manpower under arms, the balance of combat equipment such as tanks, the balance of force readiness and mobilization potential, or the balance as measured by likely war outcomes if deterrence fails. This report is concerned primarily with the last of these, and with the challenge of addressing that warfighting balance in the face of massive uncertainty rendering it meaningless to talk about allegedly best estimate scenarios.